Petroleum oil and its related products, when spilled on a water surface, initially form compact spots of concentric or irregular shapes. The petroleum-based contaminants then spread, are weathered and lose their more volatile components through evaporation and through processing by the hydrocarbonoclastic microorganisms, thus thicken and become more viscous and therefore are more difficult to separate from water.
A widely applied separation technique is based on application of separators—mechanical devices that suck a rather thin surface layer of water with oil, then depose that liquid in their tank for a duration of time necessary to allow the oil to concentrate on the surface of the tank due to the oil buoyancy, then collecting the top layer containing most of the oil for recycling or disposal, while the remaining solution is returned to the water or is also collected and dumped in designated areas.
Another approach consists in collecting oil by a broad variety of oil-absorbing materials that may differ in their selectivity in oil uptake vs. water uptake. In order for such absorption to occur, oil must come into direct contact with the absorbing material. The contact may be achieved e.g. by covering the entire oil spot with such a material, e.g. straw of hay, or some powdered porous hydrophobic inorganic or organic materials, or a compact piece of such a material, e.g. in the form of a boom, or raft, may be mechanically moved across the oil spot, absorbing oil on its way. After the oil uptake, these oil-loaded materials must be collected and are commonly removed and discarded as hazardous waste.
Neither of these approaches provides for an efficient and complete separation of oil from water. In mechanical separators, oil is inevitably collected together with a certain amount of water, and/or the water is not completely freed of oil. In collecting oil by pieces of absorbing material mechanically moved, e.g. by a power boat, it is difficult, if not impossible, to achieve total collection of oil, due to disturbance that such a motion creates on the surface of water, thus interfering with a complete absorption of oil.